External fixation devices typically include external frames which are used in conjunction with bone pins which are surgically placed into the bone fragments to be repaired or immobilized to promote healing. Such external fixation systems allow for particular placement of the bone pins that hold the bone fragment into which they are inserted, as there are often characteristics in the affected area that require delicate placement of pins such as proximity to nerves or arteries, or around joints.
The field of external fixation varies widely comprising many different types of apparatus. Typically bone pins are inserted through the soft tissue into the bone fragments; devices are then affixed to the pins and serve to connect the various fragments in such a way as to maintain correct anatomic position during the healing process. As is often the case, placement of the bone pins must be carefully selected to avoid damaging structures such as blood vessels, nerves, tendons, etc. Additionally consideration must be given to the structural integrity of the bone stock in combination with geometric stability considerations of the final construct.
Earlier fixation systems required prior assembly (complete or partial). One such know system uses a clamping type articulation element for the relative positioning of fixation bars and bone pins of an external fixation device. A drawback of such a device is that the articulation elements require being mounted on the fixation bars or the pins via their ends in advance of attachment. Until the clamping mechanism in such device is locked, the components are not held firmly and can move with respect to one another, making it difficult to hold all components in their proper relative orientation prior to or during final locking of the device.
Other known external fixation devices have been developed where the components of the external fixation devices are assembled after the pins have been inserted into the optimum position with respect to the bone fragment to be held and the tissue surrounding them. Typically the fixation bars making up the frame of the fixation device are then placed between the bone pins and articulation elements are used to connect the bars and the pins. It is often necessary that these articulation elements allow the bars or pins to be held at variable angles.
To address these concerns while providing maximum utility and ease of use; manufacturers have sought to develop fixation devices that allow for the placement of the bone pins to be independent of the external supporting structures that ultimately connect the pins together. In complex fractures there is often a need to place many pins, either because there are many fragments, or due to the poor quality of the bone and/or the nature of the fracture. Many pins, many of which will present themselves in different attitudes and inclinations, create a challenge when it comes time to connect them to the external structure. For this reason it is imperative to provide as many degrees of freedom when constructing an articulation element.
A simple analog to this condition is the case of the simply supported beam having more than 2 simple supports. This condition is characterized as being overly constrained. Unless the beam or the foundation is of sufficient flexibility it is impossible to share the load evenly across all of the multiple supports. To provide a flexible beam would defeat to goal of external fixation; we therefore need to provide a support system in the way of articulation elements that have the placement flexibility to evenly support the external structure.
One known prior art system describes an articulation component having two cylindrical joints with a revolute joint interposed between them. The characteristic mobility of this construct is two translations and three rotations for a total of five degrees of freedom. A known such system describes a similar device with the exception that the interposed revolute joint is replaced with a spherical joint. The characteristic mobility of this construct is two translations and five rotations for a total of 7 degrees of freedom. One could argue that more than 6 degrees of freedom is redundant and to some extent that is true but these extra degrees of freedom, redundant or not, make the placement of the device more flexible. This added spherical joint does however come at some expense, namely the pin to bar centerline distance must be increased, which makes for a bulkier construct while at the same time increasing the moment loading on the joints themselves.
The foregoing is believed to describe prior art systems as set forth, for example, in U.S. Pat. Nos. 6,080,153 and 7,048,735 (so called Jet-X Unilateral Fixation System), and EP 0321472.